The present invention relates to a method and to an apparatus for studying the effect of a supercritical fluid on the transition of a material from one of two condensed phases to the other and to their application to the case of a polymer material.
Supercritical fluids, which are neither gases nor liquids and which may be progressively compressed from a low density to a high density, are growing in importance as solvents and reaction media, especially in the chemical industry, in the pharmaceutical industry and in the food industry.
An example in which these fluids are applied in the chemical industry is especially that of polymers, the supercritical fluids of which allow the molecular weight and the morphology to be controlled, resulting in novel modified products.
Such modifications are often carried out by compressed-gas absorption or by recrystallization from solutions prepared with various liquid solvents.
In the case of modifications with gas absorption, the semicrystalline substance becomes more plastic and, for example, its glass transition may be lowered by several tens of degrees. The main drawback of such a technique is that this property is observed in practice only when the modified substance is in the presence of compressed gases. This greatly limits the use of this technique.
In the case of modifications by recrystallization from a liquid solution, the morphology of the condensed phase may be changed, but it is very often difficult to find suitable liquid solvents. This technique is therefore limited in practice to systems for which the solubilities are well known. In addition, the modified condensed phase must be suitably dried and this is a disadvantage, especially from the energy standpoint.
Although the scientific literature also reports various experimental studies involving a supercritical fluid on particular materials under specific conditions, it is apparent, however, that hitherto no apparatus has existed which allows the effect of a supercritical fluid on the transition of any material from one of two condensed phases to the other, by controlling all the significant parameters, to be fully studied.
The subject of the present invention is a method and an apparatus making it possible in particular to study the modification of the morphology of a substance solely by melting and recrystallizing the substance or simply by making it solidify from the fluid or liquid state under the pressure (at saturation) of a supercritical fluid under well-defined isothermal or isobaric conditions.
Typically, the pressure range may go up to at least 400 MPa and the temperature range may cover at least the 220-570 K interval.
The apparatus which allows the effect of a supercritical fluid on the transition of a material from one of two condensed phases to the other to be studied, comprises, according to the invention:
a cell (1) suitable for containing a specimen of the material to be studied and for withstanding the pressures and temperatures involved in the study;
a source of supercritical fluid and a pipe connecting this source to the inside of the cell for controlled introduction of this fluid into the cell so that it is in contact with the control specimen; means for varying, continuously or in steps, according to a defined program, the value of a first parameter chosen by the pressure (P) of the supercritical fluid, the temperature (T) of the cell and the volume (V) of the specimen in the cell, whilst keeping a second of said parameters at a chosen value, so as to induce the recording transition;
means for recording the variation in the first parameter, the variation in the third parameter and the variations (xcex94H) in the heat flux in the cell;
means for making analog recordings under similar conditions with a neutral fluid instead of the supercritical fluid.
In preferred embodiments, the apparatus of the invention also has one or more of the following characteristics:
the cell contains an open ampule which receives the specimen;
the ampule is one with a flexible wall;
the apparatus includes a high-pressure pump whose piston is actuated by a stepper motor, means for transmitting the pressure exerted by this pump to the supercritical fluid and means for controlling the stepper motor and for recording the variations in the number of steps of the motor;
a duct connects the pressure outlet of the pump to said pipe and contains the neutral fluid so that the pressure of the pump is transmitted to the supercritical fluid via the neutral fluid;
said duct includes, at the outlet of the pump, a duct part which lies upstream of the neutral fluid contained in the duct and contains a hydraulic fluid;
said duct also terminates in another cell identical to said cell so that the neutral liquid is introduced into this cell;
the cell or each cell is placed in a calorimetric detector surrounded by a thermostat;
a control and recording unit is connected to the thermostat, to the calorimetric detector, to the stepper motor and to a pressure sensor which receives the pressure exerted by the pump.
The subject of the invention is also a method for studying the effect of a supercritical fluid on the transition of a material from one of two condensed phases to the other, in which:
a specimen of the material and the supercritical fluid are introduced into a cell so that the fluid is in contact with the specimen;
the transition is induced by varying, continuously or in steps, according to a defined program, the value of a first parameter chosen by the pressure (P) of the fluid, the temperature (T) of the cell and the volume (V) of the specimen in the cell, whilst keeping a second of said parameters at a chosen value;
the variation in the first parameter, the variation in the third parameter and the variation in the heat flux in the cell are recorded;
the above operations are repeated with the same program parameter and the same chosen value but using a neutral fluid instead of the supercritical fluid, by bringing or not bringing the neutral fluid into contact with the specimen depending on the case;
and the results obtained for the two fluids are compared in order to evaluate the effect of the supercritical fluid on the transition conditions.
In preferred embodiments, the method of the invention also has one or more of the following characteristics:
a supercritical fluid chosen from carbon dioxide, nitrogen, methane, ethane, propane, or mixtures thereof, or any other fluid capable of being brought into the supercritical state, is used;
mercury is used as the neutral fluid;
the pressure of a hydraulic fluid actuated by a piston is transmitted to the supercritical fluid;
this transmission is carried out by means of the neutral fluid;
said hydraulic fluid is used to push the neutral fluid right into the cell;
said piston is displaced by the action of a stepper motor in order to create said pressure and the number of steps of the motor needed to keep the pressure at a chosen value is counted so as to determine the variation in the volume of the specimen in the cell during the transition;
the following steps are carried out: a) introduction of the supercritical fluid into the experimental cell at the pressure needed to make a high-pressure pump work; b) compression of the supercritical fluid by the high-pressure pump to the desired pressure; c) isobaric initiation of the melting of the material by controlled increase in the temperature and simultaneous recording of the changes in the volume and in the heat flux; d) isobaric initiation of the isobaric crystallization of the material by controlled reduction in the temperature and simultaneous recording of the changes in the volume and in the heat flux;
the following steps are carried out: a) introduction of the supercritical fluid into the experimental cell containing the material at the pressure needed to make the high-pressure pump work; b) compression of the supercritical fluid by the high-pressure pump to the desired pressure; c) isothermal initiation of the melting of the substance by controlled reduction in the pressure and simultaneous recording of the changes in the volume and in the heat flux; d) isothermal initiation of the isothermal crystallization of the material by controlled increase in the pressure and simultaneous recording of the changes in the volume and in the heat flux.
said material in the fluid or liquid state is modified by reducing its temperature below the controlled pressure of the supercritical fluid.
Among the applications of the invention, mention may especially be made of the study of polymer materials, in particular for determining when the supercritical fluid/polymer solution is in an equilibrium state and when this solution becomes saturated.
In particular embodiments:
the transition of the polymer is induced by varying the temperature at constant pressure;
under the same pressure conditions, a slow heating is effected in order to obtain a homogeneous, saturated supercritical gas/polymer system followed by a slow cooling in order to make a microfoam or nanofoam phase condense, whilst recording all the parameters (P,T, xcex94H and xcex94V) of the observed transitions;
the transition of the polymer is induced by varying the pressure at constant temperature;
under the same pressure conditions, a slow heating is effected in order to obtain a homogeneous, saturated supercritical gas/polymer system followed by a slow compression in order to make a microfoam or nanofoam phase condense, whilst recording all the parameters (P,T,xcex94H andxcex94V) of the observed transitions;
the stable shape of the thermogram, which does not change during the successive transitions under the atmosphere of the supercritical gas, is the indicator of the end of formation of the novel microfoam or nanofoam phase.